Water repellent aramids

ABSTRACT

A process is disclosed for rendering shaped aramid and polybenzimidazole articles water repellent by grafting fluorinated compounds to the surface of the articles. The resulting grafted polymer and shaped articles made from the grafted polymer are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polymer and shaped aramid andpolybenzimidazole articles which exhibit an inherent water repellencyand to a process for making such inherently water repellent aramid andpolybenzimidazole articles. The articles include fibers; and the processfor increasing the water repellency of the articles includes a reactionwherein fluoroalkyl groups are grafted onto the surface of the polymer.

2. Description of the Prior Art

Japanese Publication (Kokai) No. 62-81426, published Apr. 14, 1987 onthe application of Asahi, discloses a process for treating aramidsurfaces to render them more hydrophobic and decrease the coefficient ofmoisture absorption. The process includes: contact of the aramid surfaceby alkali metal hydrocarbyls or alkaline earth metal hydrocarbyls in nosolvent or in a solvent specifically required to be less polar thandimethyl sulfoxide; and then contact of the metallated aramid by ahalogenated aliphatic compound in no solvent or in a solventspecifically required to be less polar than dimethyl sulfoxide. There isno mention of the use of fluoroalkyl-containing alkylating agents.

Japanese Patent No. Sho 62-91542, published Apr. 27, 1987 on theapplication of Asahi, discloses a process for treating aramid surfacesto render them more hydrophilic and increase the coefficient of moistureabsorption. The process includes: contact of the aramid surface byalkali metal-containing or alkaline earth metal-containing compounds ina solvent specifically required to be at least as polar as dimethylsulfoxide; and then, reacting the aramid surface with water or otherpolar compound.

Japanese Publication (Kokai) No. 62-110,968, published May 22, 1987,discloses grafting materials to aramid fibers, through the use ofepoxide-functional grafting agents, to create a surface with reactiveepoxide groups.

SUMMARY OF THE INVENTION

The present invention provides a process for increasing the waterrepellency of shaped aramid and polybenzimidazole articles comprisingthe steps of: contacting the articles with a solution of an alkylsulfoxide solvent, a base whose conjugate acid has a pK_(a) in the alkylsulfoxide solvent of at least 19, and a lithium salt; maintaining thecontact for a time sufficient to swell the surface of the articles andform anions on polymer molecules on the surface of the articles;optionally, rinsing the articles to remove the base and excess lithiumsalt; and, then, contacting the articles with a fluoroalkyl-containingalkylating agent such as a perfluoroalkyl benzyl halide or a perfluoroalkyl epoxide. The lithium ion has been found to prevent individualarticles from adhering to each other.

The fluoroalkyl-containing alkylating agent is

R'(CH₂)_(m) X wherein:

R' is a perfluoroalkyl having 1-20 carbon atoms, straight-chain orbranched, and substituted by one or more ether oxygens or not;

m is 0-6; and

X is --CH[O]CH₂, --OCH₂ CH[O]CH₂, or --C₆ H₄ CH₂ Y wherein Y is Br, Cl,I, CH₃ C₆ H₄ SO₃, or CF₃ SO₃.

The process utilizes base in a concentration of 0.0001 to 6 molar in thesolution and lithium salt in a concentration of 0.0001 to 6 molar in thesolution. The preferred concentration range for both materials is from0.1 to 1.0 molar in the solution.

The solvent for the solution is generally an alkyl sulfoxide such asdimethyl sulfoxide or tetramethylene sulfoxide or combinations of thosesolvents alone or combined with polar non-protic solvents. The base isgenerally potassium tert-butoxide, sodium methoxide, potassium hydride,sodium hydride, potassium hydroxide, or sodium amide; and the lithiumsalt is generally lithium nitrate, lithium iodide, lithium chloride, andthe like provided only that the salt is soluble and does not have acidiccounterions. The base and the lithium salt can be constituted from asingle material. For example LiN(Me)₂ (lithium dimethyl amide) is both,adequate base and soluble lithium salt.

This invention also provides aramid and polybenzimidazole polymers withfluoro alkyl moieties grafted thereto and shaped articles containingsuch polymers.

DETAILED DESCRIPTION OF THE INVENTION

Aramid fibers and other shaped aramid articles have found many importantuses requiring high strength, high modulus, high heat resistance, andother similar high performance qualities. It is sometimes desirable forthe shaped aramid articles to exhibit a durable water repellent surface.This invention provides shaped aramid articles with a water repellentsurface which is an integral part of the aramid molecular surface of thearticle.

"Shaped articles", as used herein, is intended to include fibers,ribbons, tapes, rods, particles, and structures made from suchmaterials. As used herein, the words "fiber", "filament", and "shapedarticles" shall all be taken to mean shaped articles. Shaped articlesalso is intended to include articles which might be molded or casthaving a geometric shape.

The present invention relates to the use of aramid and polybenzimidazolepolymer materials.

By "aramid" is meant a polyamide wherein at least 85% of the amide(--CO--NH--) linkages are attached directly to two aromatic rings.Suitable aramid fibers are described in Man-Made Fibers--Science andTechnology, Volume 2, Section titled "Fiber-Forming AromaticPolyamides", page 297, W. Black et al., Interscience Publishers, 1968.Aramid fibers are, also, disclosed in U.S. Pat. Nos. 4,172,938;3,869,429; 3,819,587; 3,673,143; 3,354,127; and 3,094,511.

Additives can be used with the aramid and it has been found that up toas much as 10 percent, by weight, of other polymeric material can beblended with the aramid or that copolymers can be used having as much as10 percent of other diamine substituted for the diamine of the aramid oras much as 10 percent of other diacid chloride substituted for thediacid chloride or the aramid.

The polymer used in fibers of this invention includes aramids andpolybenzimidazole and is a polymer having at least one repeating unitselected from the group: --NHRCO--, R¹ --NHCONH--or ##STR1## wherein

R is selected from R³ and R¹ NHCOR²,

R¹ and R³, individually, are selected from m-phenylene, p-phenylene,3,3'-biphenylene, 3,4'-biphenylene, 4,4'-biphenylene and4,4'-diphenylene ether,

R² is selected from R¹ an (CH₂)_(x) --,

Ar is a trisubstituted aromatic radical in which two of the threeradical bonds are adjacent to one another and attached to nitrogenatoms, and

x is 1 to 10, preferably 1 to 4; said repeating unit further optionallysubstituted with halogen or C_(y) H_(2y+1) where y is 1 to 10.

The process of the present invention includes grafting an accessiblenitrogen anion to a fluoroalkyl-containing alkylating agent. Theaccessible nitrogen anions can be in solution or solvent swollen or, insome cases, merely wetted by the liquid solution of alkylating agent.The process of the present invention more specifically includes swellingthe polymer, creating anionic reactive sites on the swollen polymer,treating the reactive polymer surface to render it non-tacky, andgrafting water repellent moieties to the anionic reactive sites.

The polymer is swollen by a highly polar solvent. Solvents which aresuitable for the invention include sulfoxides such as R⁴ SOR⁵ wherein R⁴and R⁵ can be the same or different or joined together to yield asolvent with a cyclic structure, and are C₁ -C₅ alkyl. The mostpreferred solvent is dimethylsulfoxide (DMSO).

Solvent and solvent mixtures which are suitable include R⁴ SOR⁵ mixedwith a polar non-protic solvent such as N-methylpyrrolidone ortetrahydrofuran. Preferred solvent mixtures contain greater than 10%DMSO. Most preferred solvent mixtures contain greater than 50% DMSO. Itis important to the present invention that the combination of base andsolvent cause swelling of the polymer, as this permits improved contactwith the reagents. Solvents and solvent combinations which causeswelling are known in the art. See U.S. Pat. No. 4,785,038.

Reactive anionic sites are created on the swollen polymer by reaction ofthe polymer with a strong base. The strength of a base useful to createanionic sites on the polymer must be greater, in the solvent systembeing used, than the strength of the nitrogen group itself in thatsolvent. For example, it is estimated that poly(paraphenyleneterephthalamide) exhibits a pK_(a) in DMSO of about 29. (SeeMacromolecules, vol 23, pp 1065-71 (1990)). As a consequence, any basewhose conjugate acid has a pK_(a) in DMSO greater than 29 should beeffective to create a full complement of anionic reactive sites on theswollen poly(paraphenylene terephthal amide) polymer. Under the sameconditions, a base whose conjugate acid has a pK_(a) of greater than 19should be effective to convert more than 50% of the amine sites toanionic reaction sites and it has been estimated that more than 50%conversion is adequate for practice of this invention. Examples of suchbases include potassium tert-butoxide (conjugate acid is tert-butylalcohol, pK_(a) =32 in DMSO), sodium methoxide (methanol, pK_(a) =29),potassium hydroxide (water, pK_(a) =32), and sodium amide (ammonia,pK_(a) =41). Such bases are also effective for deprotonation ofpoly(meta-phenylene isophthalamide) and poly(1,4-benzamide) in DMSO. Asis understood, the relative strengths of acids and bases may differ withdifferent solvent systems, thus changing the adequacy or eligibility ofcertain bases for use in practice of this invention. A convenientlisting of equilibrium acidities for materials in DMSO can be found inAccounts of Chemical Research, Vol. 21, pp. 456-463 (1988).

While bases can be used at any concentration for some benefit, it hasgenerally been found that at least 1 mole of the base should be presentin the system for each 100 moles of accessible amide anions, as a lowerlimit. As much as one mole of base for each mole of amide anions can beused, with very little benefit for base concentrations in excess oftheir solubility limit in the system.

Salts of lithium have been discovered to alter the swollen anionicpolymer surface in such a way as to render it non-tacky and permittreatment of closely adjacent surfaces without adherence of the polymerarticles to themselves and each other. While the reason for the effectof the lithium salts is not entirely understood, it is believed that anylithium salt which has a non-acidic counterion and is soluble in thesolvent system is eligible for use in the practice of the presentinvention. Polymer surfaces, once swollen by the sulfoxide solvents andreacted by the added bases, tend to become tacky and stick together evenwhile immersed in the solvent system; for example, as described in U.S.Pat. No. 4,889,579. It has now been discovered that addition of asoluble lithium salt to the system causes an alteration in the surfaceof the swollen polymer such that the reactive sites remain available forthe up-coming grafting reaction but the aramid article, itself, will notreadily adhere to neighboring articles. Non-tacky surfaces areespecially important to prevent filament-to-filament adhesion in yarns.

The concentration of lithium salt to be used in the practice of thisinvention can vary from as little as 1:1 to as much at 2:1 or more, bymole, of lithium to base, to obtain adequate reduction in tackiness ofthe aramid substrate. Less lithium can be used if tackiness isunimportant or less important for any reason. Beyond a mole ratio of 2:1for lithium to base, there is very little improvement in effect.

The fluoroalkyl alkylating agents which are grafted to the anionicaramid sites include perfluoroalkyl benzylhalides, fluoroalkyl epoxides,and the like in conformance with the following formula:

R'(CH₂)_(m) X wherein:

R' is a perfluoroalkyl having 1-20 carbon atoms, straight-chain orbranched, and substituted by one or more ether oxygens or not;

m is 0-6; and

X is --CH[O]CH₂, --OCH₂ CH[O]CH₂, or --C₆ H₄ CH₂ Y wherein Y is Br, Cl,I, CH₃ C₆ H₄ SO₃, or CF₃ SO₃.

A broad range of fluoroalkyl alkylating agent concentrations can be usedin the practice of this invention. For example, at one extreme, aconcentration of as little as 0.01 mole of agent per mole of amide anionhas been found to yield a convenient reaction rate; and, at the otherextreme, liquid fluoroalkyl alkylating agents can be used neat, withoutsolvent. Too low a concentration of the alkylating agent causesincomplete or slow reaction; and when the concentration is too high,precipitation of the agent may occur. Too low a concentration of thealkylating agent causes inadequate surface fluorine content.

The polymer of this invention, which results from grafting fluoro alkylmoieties to the aforedefined aramid, can be in the form of a shapedarticle and has at least one repeating unit selected from, ##STR2##wherein

R is selected from R³ and R¹ NHCOR²,

R¹ and R³, individually, are selected from m-phenylene, p-phenylene,3,3'-biphenylene, 3,4'-biphenylene, 4,4'-biphenylene and4,4'-diphenylene ether,

R² is selected from R¹ and (CH₂)₂ --,

Ar is a trisubstituted aromatic radical in which two of the threeradical bonds are adjacent to one another and attached to nitrogenatoms,

x is 1 to 10, preferably 1 to 4; said repeating unit further optionallysubstituted with halogen or C_(y) H₂₊₁ where y is 1 to 10. and Z is

R'(CH₂)_(m) X wherein:

R' is a perfluoroalkyl having 1-20 carbon atoms, straight-chain orbranched, and substituted by one or more ether oxygens or not;

m is 0-6; and

X is ##STR3## or --C₆ H₄ CH₂ --.

X-ray photoelectron spectroscopy studies have shown that the surfaces ofthe fibers of poly(p-phenylene terephthalamide), after the alkylatingtreatment herein described, may have 1-8 atom percent fluorine and 2-4atom percent CF₃. Those studies evidence that the R'(CH₂)_(m) groups arecovalently bonded to the polymer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

In this example two types of para-aramid fabrics were treated by theprocess of this invention and then were tested for water repellency andthe durability thereof. Fabric A in this example was a fabric utilizedin fire fighting jackets and made from the para-aramid fiber sold by E.I. du Pont de Nemours and Company under the trademark designationKevlar® 100. Fabric B was a fabric utilized in ballistics garments andsold by E. I. du Pont de Nemours and Company under the trademarkdesignation Kevlar® 29. Fabric A was a 6 oz/yd² plain weave, 35×35 yarnends per inch, made from 660 denier yarn with 440 filaments. Fabric Bwas a 12 oz/yd² plain weave, 31×31 yarn ends per inch, made from 1000denier yarn with 667 filaments. Each fabric was subjected to threetests, two utilizing fluoroalkyl alkylating agents within the embrace ofthis invention and one utilizing commercially available water repellencytreatment as a control. In one of the tests utilizing the process ofthis invention, the fluoroalkyl alkylating agent was paraperfluorooctylbenzyl bromide; and, in the second test, the fluoroalkylating agent waspara-trifluoromethyl benzyl bromide.

To treat the fabrics, 14.5 grams of a fabric were soaked for ten minutesin 30 ml of DMSO having a 0.20 molar concentration of potassiumtert-butoxide and a 0.35 molar concentration of lithium nitrate at 25°C. The fabric was rinsed in DMSO and was then soaked for ten minutes in300 ml of a solution containing 37.6 grams of the fluoroalkyl alkylatingagent in 50:50, weight, DMSO and tetrahydrofuran. The fabric was rinsedin water and dried. The fabric exhibited excellent hydrophobicity.

The fabrics, once treated, were subjected to a laundering treatmentwhich consisted of five consecutive washing cycles. Each cycle includeda 12-minute agitated machine wash in water at a temperature of 105° F.with detergent, followed by a machine rinse and spin, and completed bytumble drying for 25 minutes at 155°-160° F.

The resulting fabrics were subjected to a Spray Test for WaterRepellency (AATCC Test Method 22-1985). Test results are shown in Table1.

                  TABLE 1                                                         ______________________________________                                                      Spray Rating                                                                  (0 is very wet; 100 is not wettable)                            Treatment Type  Before Wash                                                                              After 5 washes                                     ______________________________________                                        Fabric A                                                                      C.sub.8 F.sub.17 -Benzyl Bromide                                                              90         70                                                 CF.sub.3 -Benzyl Bromide                                                                      70         50                                                 Commercial      90-100     50                                                 treatment                                                                     Untreated        0          0                                                 Fabric B                                                                      C.sub.8 F.sub.17 -Benzyl Bromide                                                              70         50                                                 CF.sub.3 -Benzyl Bromide                                                                      70          0                                                 Commercial      100        90                                                 treatment                                                                     Untreated       50          0                                                 ______________________________________                                    

The commercially available water repellency treatment comprises coatingthe fabric by an aqueous dispersion of a hydrocarbon wax, a mixture ofpoly(perfluoroalkyl methacrylates), and an emulsifier; and, then, curingthe material for a few minutes at about 380° degrees F.

Example 2

In this example, an additional sample of Fabric B from Example 1 wastreated utilizing an epoxide-functional fluoroalkyl alkylating agent.The treatment and the treatment conditions were the same as were used inExample 1. The epoxide-functional alkylating agent was CF₃ --(CF₂)₅--CH₂ --CH[0]CH₂ /CF₃ --(CF₂)₇ --CH₂ --CH[O]CH₂ /CF₃ --(CF₂)₉ --CH₂--CH[O]CH₂ sold by E. I. du Pont de Nemours and Company under thetrademark designation Zonyl® TE.

The fabrics were tested in accordance with MIL Spec MIL-C-44050A beforewashing and after washing. Results are shown in Table 2. The treatmentof this invention resulted in a fabric exhibiting outstanding, durable,water repellency.

                  TABLE 2                                                         ______________________________________                                                     This                                                                          Inven-   Military   Commercial                                   Test         tion     Spec       treatment                                    ______________________________________                                        BEFORE WASH                                                                   Water Spray Rating                                                                         100      >/= 90     100                                          Oil Repellency Rating                                                                       6       >/= 5       5                                           (higher is better)                                                            Dynamic Absorption                                                                          8.2%    </= 15%     8-10%                                       (of water)                                                                    AFTER WASH                                                                    Water Spray Rating                                                                          90      >/= 80      80                                          Oil Repellency Rating                                                                       6       >/= 5       5                                           (higher is better)                                                            Dynamic Absorption                                                                         11.2%    </= 15%    10-12%                                       ______________________________________                                    

The shaped articles of this invention include the polymer describedabove with fluoroalkyl groups grafted thereto. The articles presentwater-repellent, oil-repellent, grafted surfaces; and, yet, the texture,the drape, and the hand of the articles is not altered from that ofsimilar articles with ungrafted surfaces. The grafted surfaces are moredurable than surfaces which have been merely coated with an agentbecause the grafted fluoroalkyl materials are covalently bonded to thepolymer substrate of the article.

We claim:
 1. A process for increasing the water repellency of shapedaramid articles selected from the group consisting of fibers, filaments,ribbons, tapes, rods, and particles while preventing the individualarticles from adhering to one another during the process comprising thesteps of:a) contacting the aramid articles with a solution of:i) analkyl sulfoxide solvent; ii) a base which has a pK_(a) in the alkylsulfoxide solvent of at least 19 in a concentration of 0.0001 to 6 molarin the solution; and iii) a lithium salt soluble in the solution in aconcentration of 0.0001 to 6 molar in the solution and at a mole ratioof the lithium with the base of at least 1:1; b) maintaining the contactfor a time sufficient to swell the aramid articles and form anions onaramid molecules on the surface of the aramid articles; c) contactingthe aramid articles with a fluoroalkyl-containing alkylating agentR'(CH₂)_(m) X wherein: R' is a perfluoroalkyl having 1-20 carbon atoms,straight-chain or branched, and substituted by one or more ether oxygensor not; m is 0-6; and X is --CH[O]CH₂, --OCH₂ CH[O]CH₂, or --C₆ H₄ CH₂CH₂ Y wherein Y is Br, Cl, I, CH₃ C₆ H₄ SO₃, or CF₃ SO₃ whereby thealkylating agent is grafted to the aramid articles.
 2. The process ofclaim 1 wherein the base is present in a concentration of 0.0001 to 6molar in the solution.
 3. The process of claim 1 wherein the lithiumsalt is present in a concentration of 0.0001 to 6 molar in the solution.4. The process of claim 1 wherein the alkyl sulfoxide solvent is R⁴ SOR⁵wherein R⁴ and R⁵ are alkyl of 1-5 carbon atoms and can be the same ordifferent or joined together to yield a cyclic-structure.
 5. The processof claim 1 wherein the base is selected from the group consisting ofpotassium tert-butoxide, sodium methoxide, potassium hydroxide, andsodium amide.
 6. The process of claim 1 wherein thefluoroalkyl-containing alkylating agent is perfluoroalkyl benzyl bromidewherein the alkyl is 1 to 10 carbon atoms.
 7. The process of claim 1wherein the fluoroalkyl-containing alkylating agent is fluoroalkylepoxides wherein the alkyl is 1 to 10 carbon atoms.
 8. The process ofclaim 1 wherein the aramid is poly(p-phenylene terphthalamide).
 9. Awater repellent shaped article made by the process of claim 1.